CF4, C2F6, NF3 and SF6 are commonly used in the semiconductor manufacturing industry, for example, in dielectric layer etching and chamber cleaning. Following the manufacturing or cleaning process there is typically a residual PFC content in the effluent gas stream pumped from the process tool. PFCs are difficult to remove from the effluent stream, and their release into the environment is undesirable because they are known to have relatively high greenhouse activity.
Plasma abatement has proved to be an effective method for degradation of PFCs to less damaging species. In the plasma abatement process, an effluent gas containing the species to be destroyed is caused to flow into a high density plasma and under the intensive conditions within the plasma the PFCs are subjected to impact with energetic electrons causing dissociation into reactive species which can combine with oxygen or hydrogen to produce relatively stable, low molecular weight by-products, for example, CO, CO2 and HF, which can then be removed in a further treatment step.
In one form of previously known plasma abatement, the plasma is a microwave plasma. An example of a microwave plasma reactor is described in UK Patent no. GB 2,273,027. In that device, a microwave plasma is generated between two electrodes located in a chamber in closely opposed relationship, with one of the electrodes having an axial hole through which an excited gaseous medium is exhaust from the chamber.
In such a reactor, the microwave radiation is conveyed into the chamber from a microwave generator by a waveguide. In order to reduce the amount of microwave radiation reflected back into the microwave generator by the waveguide, the waveguide is provided with one or more tuning screws extending substantially orthogonally thereinto. By adjusting the amount by which each of the screws extends into the waveguide, the amount of microwave radiation reflected back into the microwave generator by the waveguide can be minimised. However, the presence of the tuning screws serves to reduce the energy absorption into the plasma.